There has been no or little wide-angle optical system designed to have a viewing angle of view exceeding 180°, and only two have been known in the art: Patent Publication 1 shows an optical system having a viewing angle of 216° and Patent Publication 2 shows an optical system having a viewing angle of 270°.
Patent Publication 1: JP(A) 10-288742
Patent Publication 2: U.S. Pat. No. 2,947,219
The above conventional optical systems having an angle of view exceeding 180° have none of f-θ performance that helps prevent an image from becoming small around the angle of view. They are not a small-sized, fast optical system either, because the F-number is large or the optical system is not fast, and the total length is long.
Having been made with such situations of the prior art in mind, an object of the present invention is to provide a fast, superwide-angle imaging or projecting optical system that, albeit being a wide-angle optical system having an angle of view exceeding 180°, is of small size and reduced F-number.